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RE: parking structure live load

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Perhaps you use the National Building Code of Canada in your work.  
The NBCC does indicate that the larger of the design snow and live 
loads applies.  This is probably appropriate in most areas of Canada. 
Unfortunately, (to my knowledge) no U.S. code indicates the 
appropriate combination of these two conditions for parking 

I have been priviledged to live in two locations where the either/or 
approach doesn't really work; in both Denver, Colorado and Seattle, 
Washington the design snow loading condition occurs due to a single 
storm (or perhaps two storms within a week of each other).  There are 
numerous other locations in the U.S. where the design condition does 
not result from long accumulation.  These single-storm loads can be 
as large as 40 psf.  If you also consider roof-level parking that is 
generally full day and night (for instance, at an airport), it is 
likely that the structure will experience the reduced live load and 
some snow load.  Because of these conditions, I've seen Denver 
parking structures that required 30 psf snow PLUS 30 psf live load 
(50 psf with 40% reduction).  In the Seattle metro area, many garages 
are design for 55 psf on the roof although some jurisdictions will 
allow design for an unreduced 50 psf; the either/or approach would 
result in design for 30 psf.  (Since Harold lives and works on the 
Colorado front range, this is probably the condition he had in mind.)

The long and short of it is that the appropriate coincident loading 
is sensitive to local climatology.  Perhaps that is why the U.S. 
codes are silent on the issue.


> From:          Paul Meyer <PMeyer(--nospam--at)>
> Subject:       RE: parking structure live load
> Date:          Mon, 16 Aug 1999 18:44:44 -0700

> This probably doesn't need repeating to the learned audience here, but you
> would use EITHER the snow or the parking load, whichever was greater.  If
> you've got almost a metre (2.5 feet) of old snow on a parking deck, you
> probably don't have a lot of cars there too.  Fresh snow is much less dense,
> and (with few exceptions) the amount that could fall on a parking deck
> already full of vehicles will be very small.  In those places where a single
> snowstorm could contribute a significant mass of snow, you are going to see
> that the snow load controls anyway, since you need to consider the snow load
> on a structure that remains unplowed all winter.
> -----Original Message-----
> From: Harold Sprague [mailto:harold.sprague(--nospam--at)]
> Sent: Monday, August 16, 1999 12:30
> To: 'seaint(--nospam--at)'
> Subject: RE: parking structure live load
> When I have done surveys, the 50 psf is adequate for passenger car loads.
> If you consider top decks you may have to add snow loading and probably
> drifting snow provisions.  Often times there are areas that 

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Michael Valley                                   E-mail: mtv(--nospam--at)
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201